The power factor (pf) is defined as the ratio of the actual power to the apparent power. Phrased differently, the power factor is the ratio of watts average power to the apparent power in an AC circuit, typically expressed as ##EQU1##
The power factor is the cosine of the phase angle between the voltage and current, ranging from 0 (purely reactive circuit) to 1 (purely resistive circuit). A power factor of less than 1 therefore indicates some component of reactive current. The greater the power factor, the greater the useful power being delivered to the load. Circuits that use only a little power are not adversely affected by a poor power factor. However, as individual circuits are combined into hundreds or thousands of circuits, poor power factor becomes a problem. Poor power factor is also a problem with circuits involving significant power output. Power utilities charge industrial users according to the power factor, which explains the capacitor yards one sees behind large factories, built to cancel the inductive reactance of motors found in machinery.
Although power generation plants generate AC power, DC power is very useful for certain applications, including many motors. An AC-DC converter takes AC power, such as found in most households, and converts it into DC power. Power factor is enhanced in high power factor converters by forcing the input current waveform to follow the input voltage waveform, which is usually sinusoidal. This technique minimizes current distortion and increases the power capacity of the standard AC outlet, since the outlets are protected by current actuated circuit breakers.
Most electronic loads and switching power supplies use a full wave bridge rectifier and capacitive input filter. Such a circuit draws power from the input line when the instantaneous AC voltage exceeds the capacitor voltage. The resulting current is distorted and has a high harmonic content. As a result, the power factor is low, typically in the range 0.5-0.6, with the input apparent power much higher than actual power. Power factor correction (PFC) circuits allow better utilization of the AC line by increasing the power factor to 0.98-0.99. In addition, the high power factor leads to the reduction of the input current harmonics. This is important in view of the latest IEC 1000-3-2 (replacing IEC 555-2) regulation, adopted as a European standard (EN 61000-3-2) mandatory for all equipment manufactured after 2000, which allows a third harmonic level of no more than 3.4 mA/watt with an absolute limit of 2.3 amps rms max for power levels above 75 W. Thus, a 100 W unit is limited to third harmonic current of 340 mA.
European power authorities are concerned about voltage distortion caused by non-linear loads. At present levels of growth of non-linear loads, they estimate that by 2001 such voltage distortion will exceed 5%. Harmonic voltage distortion is more of a European problem rather than a North American problem due to the differences in design of the respective distribution systems. Delta-wye distribution transformers tend to dissipate the triplen harmonics, thus preventing them from causing voltage distortion on the primary side of the transformers. Since the European distribution system uses a few large distribution transformers with very large loads, large motors can be on the same circuit as electronic loads and thus be subject to the harmonic voltage distortion caused by the electronic loads. In the North American distribution system, a large number of small distribution transformers are used with relatively small loads. Large motors are driven from higher voltages than electronic loads, so they are isolated from the sources of voltage distortion. Nevertheless, the triplen harmonics dissipate in delta-wye distribution transformers as unwanted heat.
PFC circuits fall into two groups: passive and active. Passive circuits are used primarily in low power applications since reactive components (chokes and capacitors) tend to be quite large. On the other hand, active circuits, such as switched-mode PFC circuits, run at high frequencies and process a lot of power at very high efficiency. The topology most commonly used is known as non-isolated boost. This topology usually utilizes a dedicated controller which performs all conversion, monitoring, and protection functions. Existing PFC circuit designs produce an output DC voltage which is higher than the peak of the input sine wave at high line.
The drawback of this approach is that when the input line produces an overvoltage spike, the boost shuts down and output is charged to the peak of the input waveform. This situation can be damaging to following stage DC--DC converters. The overload or short circuit conditions in current designs cannot be controlled, with the input fuse usually clearing, which can be objectionable in some critical applications. Upon initial application of AC power, the inrush current can be very high, creating disturbance of the input sine wave and causing the protection fuse to blow.
Commonly used passive protection devices such as thermisters provide only limited protection against inrush current, while providing no protection against short circuit and overvoltage conditions. Conventional buck-boost converters use power FET's (field effect transistors) to switch at high frequencies in order to regulate the output voltage. These devices are relatively expensive and fragile compared to SCR's (silicon controlled rectifiers).
In addition, the series switch of the conventional buck-boost converter is an extra power switch which produces still more objectionable losses. The fast switching frequency produces still more losses.